The present invention relates generally to converting energy into useful forms, and particularly, to converting energy into useful forms by stimulating high peak reaction rates of short duration in chemical reactions in a regional volume.
A method to convert chemical energy into electricity uses chemical reactions such as fuel-air reactions to create non-equilibrium concentrations of highly vibrationally excited products. When those products migrate to a conducting surface, the products may generate hot electrons in the surface. When the conducting surface is sufficiently thin, a form of semiconductor device may collect the hot electrons and convert them into electricity.
Another method to convert chemical energy into electricity uses chemical reactions such as a fuel-air reaction to create photon radiation with energies characteristic of the reaction temperatures. Because the photon energies of typical reaction temperatures lie within the range of photovoltaic semiconductor converters, photons may be converted into electricity using photovoltaic means. A portable thermo-photovoltaic power source is described in the U.S. Pat. No. 5,593,509.
The efficiency of a known device that stimulates reactions increases with the temperature of the device. The efficiency of the device to collect and convert the reactants to electricity, however, decreases sharply as the operating temperature increases beyond ambient temperature, which for typical semiconductors is about 300 to 400 degree Kelvin.
It is, therefore, highly desirable to operate an energy collecting and converting device at an ambient temperature while operating the chemical reactions that generate vibrationally excited specie at the maximum possible temperature and reaction rate.
Pulsed chemical reactions cause maximum reaction rate and permit a device, e.g., a semiconductor device, to operate up to its highest allowable operating temperature. The thermal mass of the semiconductor delays and minimizes significant heating. During the period of time between pulses, also referred to as a dead time, the energy collecting device may dissipate the heat generated during the pulsed reaction time and peak power portion of the reaction-collection cycle.
Therefore, it is also desirable to have pulsed chemical reactions that generate the highest possible peak power and peak reaction rates to produce hot electrons.
A problem associated with pulsed chemical reactions is initiating and sustaining the reactions. The problem becomes more severe when the reaction occurs near a surface, e.g., a conducting surface, and the reaction is confined to a small volume, e.g., when dimensions of the confining volume are the order of a micron or less.
In a typical reaction chamber, the energy of reactions is contained in and by the gas and contributes to sustaining the reaction above ignition. In such chambers, the reaction is sustained or maintained in part because reaction intermediates called autocatalysts are created and consumed by the reaction.
In a small reaction region, however, sustaining or maintaining the reaction becomes problematic. For example, in a small reaction region, any electromagnetic energy generated radiates promptly out of the small reaction region because of the extremely high surface to volume ratio of the small, micron-size region. Further, translational and vibrational energy of autocatalysts and reaction products created as a result of the reactions is quickly dissipated upon contact with the reaction chamber walls. The energy generated in smaller volumes, therefore, do not contribute significantly to sustaining the reaction.
In small reaction volumes, e.g., with dimensions in the order of one micron, the burst of reactions is a transient phenomenon that is extinguished when the reactions deplete the autocatalysts. These auto-catalysts, which are generated during the reaction, play a key role in sustaining the reaction. One micron is the size of approximately two diffusion lengths for vibrationally excited byproducts of a typical fuel-air reaction during their 10 nanoseconds lifetime after initiation. The vibrationally excited species thermalize during this time. This diffusion length is typically longer than the translational mean free path and is a function of the lifetime of the vibration state.
Fast depletion of the energy of reaction tends to extinguish the reaction. Reactions such as fuel-oxidizer reactions are maintained by the creation of autocatalysts. The autocatalysts are consumed by reaction with the fuel and oxidizer and are produced as a result of the reactions. Keeping or raising the gas temperature above the ignition temperature serves to create the autocatalysts.
The autocatalyst are typically free radicals that are known to sustain a reaction and rapidly drive the reaction to completion. Therefore, it would be advantageous to have a method to insert the autocatalysts into the reacting mixture. By introducing more autocatalysts in the reaction, the reaction can be sustained beyond its natural tendency to deplete energy and become extinguished.
The byproducts of a reaction in the small reaction region are initially created in highly vibrationally excited states of gas molecules. It has been observed that vibrationally excited species may collide between hundreds or thousands of times with other specie in the gas before the energy is dissipated into the gas, such as into translation and rotation modes. If, e.g., a vibrationally excited state would take about 100 collisions to thermalize, the lifetime would be in the order of 10 nanoseconds. When such vibrationally excited species diffuse through the gas and contact a metal surface, it has been demonstrated that they may transfer a major fraction of their energy during a single collision with the surface and in the form of a hot electron. This electron energy transfer may also take away energy from the reaction in the small reaction region forming a micro volume, and deny the reaction the energy needed to maintain the temperature of reaction. Instead, the energy denied to the reaction is transferred to the surface.
These forms of transferred energy, e.g., radiation and hot electrons, may be collected using semiconductor devices. The same semiconductor devices may also convert the energy into more useful forms such as electricity.
It is well known that the reaction surface chemical reactivity increases almost exponentially with increase in temperature. It would be highly desirable to have only the reaction surface reach the high temperature, so that only minimum amount of heat is used to raise the reaction surface temperature.
It is also desirable to have the thinnest possible reaction surface that switches to a high temperature for a short duration, and to have the reaction surface reach this temperature when the reactants are in contact with the reaction surface. Further, to efficiently stimulate and generate energy, it is desirable to concentrate the energy used to heat the reaction surface into pulses. Yet further, it is desirable to initiate reactions in pulses.
Molecules collide with the device's surface and also produce a pulse of heat. This pulse of heat, injected into the device surface, is transient and therefore, the device may dissipate the heat into the device's volume over time. In this way, the device operates at its average temperature, not the peak temperature of the reaction. This mode operation reduces or eliminates high temperature that would normally cause the device to run inefficiently.
A pulse of one electron volt hot electrons lasting under 500 femtoseconds, when injected into a thin metal conductor by any one of many known external means may concentrate the electron energy in the conductor surface electrons as a result of the hot electron transfer. This concentration raises the temperature of the surface electrons to exceed approximately 5,000 Kelvin within approximately one picosecond and forms hot electron gas. The hot electron transfer and the raising of the temperature occur in a conductor having a dimension of order of the diffusion length for 1 eV hot electrons. This diffusion length is in the order of 10 nanometers, which is typically 30 molecular or atomic layers thick.
It has been shown that this hot electron gas may react within picoseconds with any chemicals adsorbed on the surface of the conductor, thereby driving reactions which may even be inaccessible to thermal processes.
It has been shown that the 5,000 Kelvin or hotter electron gas couples to the metal vibrations, also referred to as phonons, to raise the temperature of the phonons to the order of 2,000 Kelvin over a similar dimension of surface and over a time period of 1–3 picoseconds. The phonons move more slowly than the electrons, and therefore dissipate their energy to and equilibrate with the bulk material over time periods typically of order 50 picoseconds.
The result of the hot electron pulse is a metal surface with an effective temperature far exceeding that of the bulk, and advantageously, a reaction surface with activity associated with the peak temperature. This high temperature may persist until the phonons couple the energy to the bulk, i.e., for about 50 picoseconds, which is the time the phonons take to couple their energy to the bulk.
Further, when there is a catalyst surface with temperature exceeding thousands of degrees Kelvin, adsorbates on that surface may promptly react or dissociate, and free radical specie and translationally hot atoms or molecules may readily and promptly desorb into the region near or on the reaction surface. Such free radicals and energetic specie are known to be autocatalytic and necessary for initiating and sustaining chemical reactions, such as combustion.
A known method to create a hot electron pulse in the surface uses femtosecond lasers. While such lasers create a short pulse, they are typically laboratory sized and cannot be reduced to micro-chip dimensions.
Flashlamps driven by pulsed electrical discharges are another known way to create and inject free radicals into a reactive chemical mix to initiate reactions. These fast flashlamp methods of causing electrical discharges, however, typically yield pulses no shorter than 5 to 10 nanoseconds, and require kilo-volt initiator and switching systems. Further, the flashlamps will only cause significant hot electron generation in the conductive surface and not in micron-sized volume of reactants. Thus, such flashlamps typically render optical stimulation inefficient.
Therefore, it would be highly desirable to have a method and device that provide a burst of hot electrons into nanometer dimension surfaces efficiently, and during the time before the phonons of the thin reaction surface reach equilibrium with the bulk.